The present invention relates to a motor having a bearing lubricated by a magnetic fluid. Such a motor may be used, for example, to rotate a polygon mirror, a magnetic disc, or an optical disc.
In JP-A-4-150753 there is disclosed a motor having a stator and a rotor, the stator having a base part and a projection projecting therefrom, and the rotor having a shaft extending into the projection. There is also a rotary bearing between the shaft and the inner wall of the projection, and the rotary bearing is lubricated by a magnetic fluid. To hold the magnetic fluid in the space between the shaft and the inner wall of the projection, a magnetic seal was provided at the end of the projection furthest from the base. An armature and drive coils respectively mounted on the rotor and stator form a drive means, so that the application of suitable drive currents to the rotor coils will cause the rotor to rotate relative to the stator about the rotation axis defined by the shaft.
In order to limit radial movement of the rotor relative to the stator, JP-A-4-150753 proposes that a magnetic thrust bearing be provided therebetween, the thrust bearing comprising first and second permanent magnets mounted respectively on the stator and rotor, and spaced in the radial direction. The polarities of the first and second permanent magnets were chosen so as generate a suitable force therebetween to resist radial movement of the rotor relative to the stator.
In JP-A-4-150753, the permanent magnet of the magnetic thrust bearing on the rotor was mounted on a radially outer surface thereof, and the permanent magnet on the stator was mounted on the inner surface of a cylindrical part of the stator which extended around the circumference of the rotor.
In the motor of JP-A-4-150753 discussed above, the fact that the stator had a cylindrical part extending circumferentially around the rotor, so that the first permanent magnet of the magnetic thrust bearing could be mounted thereon, had the effect of increasing the transverse dimensions of the motor. Therefore, consideration has been given to arrangements in which the rotor has an annular part extending around the projection of the stator which receives the shaft, so that the magnetic bearing may then be located between that projection and the annular part of the rotor.
In co-pending U.S. patent application Ser. No. 08/114176 (and corresponding European Application No. 93306377.8) a motor was described in which the magnetic thrust bearing was between the projection which received the rotor shaft and an annular part of the rotor surrounding that projection, with the magnetic thrust bearing being positioned radially outwardly of the magnetic seal and being axially aligned therewith.
However, it has been found that such an arrangement is not wholly satisfactory. The positioning of the magnetic thrust bearing and the magnetic seal generates magnetic flux therebetween, and the path of at least part of that flux is in the gap axially beyond the magnetic seal relative to the rotary bearing. As a result, forces are generated which act on the magnetic fluid, which is lubricating the rotary bearing. Such forces diminish the effect of the magnetic seal, increasing the risk of leakage of the magnetic fluid. This is undesirable because leakage of magnetic fluid may result in contamination of other components, such as a polygon mirror mounted on the rotor.